The present invention relates to a MOSFET control circuit for a dual winding generator.
Lundell-type electric generators have long been used in the automotive industry to provide electrical power for automobiles and trucks. The typical automotive generator today is a self-excited three phase synchronous Lundell-type generator. These generators internally comprise parallel sets of three phase windings. Due to consumer demand for comfort and convenience and complex control systems required to meet governmental regulation, electrical demands on modern automobiles have increased substantially over the years.
These generators must provide sufficient current at both high and low engine speeds. The current which such a generator can produce with the engine running at very high speeds is limited by the number of winding turns in the coils. For a generator to produce a high level of current at very high engine speeds, its coil must only have a few winding turns. The coil must have a larger number of winding turns, however, to produce sufficient current at low engine speeds.
To design generators for motor vehicles capable of producing high levels of current at high engine speeds but still capable of producing sufficiently high levels of current at low engine speeds, the coil can be divided into two windings connected in series at low engine speeds and in parallel at higher engine speeds. The prior art teaches the use of circuits containing thyristors, such as silicon controlled rectifiers (xe2x80x9cSCRsxe2x80x9d), to allow the windings of the generator to function as if they were connected in series or in parallel. See German Patent DE 32 27 602 A1 to Marelli Autronica S.p.A., Alternating Current Generator, Particularly for Motor Vehicles, which is hereby incorporated by reference. Thyristor circuits allow the windings to operate in series at lower speeds and in parallel at higher speeds.
Thyristor circuits, however, have their own disadvantages. First, thyristors are limited in operation up to temperatures of 125xc2x0 C. and typically they do not operate at higher temperatures without substantial derating of their operational limits. Second, thyristors allow current to flow in only one direction in the circuit and thus do not permit control of overvoltage, as they cannot be used as possible bypassing bidirectional devices to limit system overvoltages that may occur due to sudden load changes. Third, thyristor circuits are less efficient than other types of circuits, such as MOSFET circuits, when operating in series. Fourth, thyristors are current-controlled and require more power to control than voltage-controlled devices, such as MOSFETs.
Accordingly, the objective of the invention is to provide a MOSFET control circuit as a substitute for thyristors for use in generators with dual windings. A control circuit in accordance with the present invention is characterized by the features specified in claim 1.
The control circuit of the present invention includes three subcircuits. The first subcircuit uses two or more MOSFETs connected between the upper and lower windings of the generator to permit the windings to operate in series or in parallel. The other two subcircuits are gate drive circuits that control the operation of the MOSFETs. One subcircuit controls when the windings operate in series or in parallel, depending on speed and load. The other subcircuit provides overvoltage control by operating the MOSFETS as diodes only in the normal mode, but permitting bidirectional current flow to prevent over-voltage resulting from sudden load changes.
The circuit of the present invention is for use with a three-phase, dual winding generator. There are therefore three sets of windings, the sets typically called phase A, phase B, and phase C. In addition, each set of windings has a lower winding and an upper winding. Thus, in the circuit, there is a lower winding and an upper winding that correspond to phase A, phase B, and phase C.
In one embodiment of the present invention, the corresponding lower and upper windings are 180 degrees out of phase with respect to each other. These lower and upper windings are connected to each other by a semiconductor switch consisting of two (MOSFETS used with the present invention may be of either the P or N channel type) MOSFETs, in which the drain terminals of the two MOSFETS are connected to each other. The gate terminal of one MOSFET is controlled by a gate drive circuit that operates the windings in series or in parallel. The gate terminal of the other MOSFET is controlled by a gate drive circuit that permits current to flow in either direction under excessive DC voltage situations while at the same time permitting current flow in only one direction (from Source to Drain Only) for operation in series connection, the normal mode of operation.
In a second embodiment of the present invention, each corresponding lower winding and upper winding is in phase with respect to each other. In this embodiment, the lower windings corresponding to two phases are connected to the upper winding corresponding to the third phase by three MOSFETs. Each of the three MOSFETs is connected to the drain of the other two MOSFETs. The gate terminal of one MOSFET is controlled by a gate drive circuit that operates the windings in series or in parallel. The gate terminals of the other two MOSFETs are controlled by gate drive circuits that permit current to flow in either direction under excessive DC voltage situations while permitting diode-like (unidirectional current flow) behavior under normal series mode of operation.